zero derivative condition

Hi Dear All,
If we have the uniform inflow velocity like ui=0.02. Then what’s the physical meaning of zero derivative condition for inlet/outlet boundary conditions that proposed by Grunau et al.

a vanishing velocity-derivative normal to the boundary is often used to simulate an infinitely large domain. it is artificial, without physical meaning. in physics the domain is much larger, but in simulation it needs to be cut somewhere. zero-gradient means that “the flow does not change any more in space”. it is used for example on outlets to reduce numerical effects like backflows which you have with velocity boundaries.